A recent approach for thrombosis has involved inhibiting thrombin enzyme activity, and compounds used for this purpose have included heparin, low molecular weight heparin, hirudin, argatroban, hirulog and the like. All such compounds inhibit the enzyme activity of thrombin, and work by inhibiting fibrin blood clot formation without specifically inhibiting the effect of thrombin on cells. Bleeding tendency is therefore a common side effect encountered in the clinic. The role of thrombin in thrombosis is not limited to its blood clotting activity, as it is believed to also participate in platelet aggregation at sites of vascular injury occurring as a result of the activation of platelet thrombin receptor.
Another approach for thrombosis has been the use of intravenous injection agents such as Abciximab, Eptifibatide and Tirofiban, as GPIIb/IIIa receptor antagonists. These compounds, while exhibiting powerful anti-thrombotic effects by suppressing platelet aggregation induced by various stimulation such as thrombin, ADP, collagen, PAF or the like, also produce a bleeding tendency as a side effect similarly to thrombin enzyme activity inhibitors. For this reason, no such compounds have yet been marketed, although their development as oral agents continues to progress.
Restenosis is a vascular hyperproliferative response to vascular wall injury induced by invasive treatment such as coronary angioplasty, and this phenomenon may be provoked by the direct or indirect effect of thrombin on cells. Platelets adhere to injured blood vessels, leading to release of growth factors and eliciting proliferation of smooth muscle cells. Smooth muscle cells may also be affected indirectly by the action of thrombin on endothelial cells. Also, platelet adhesion occurs and procoagulant activity increases at sites of vascular injury. Smooth muscle cells can undergo further direct stimulation due to the high local thrombin concentration which is produced at such sites. While recent studies using the powerful thrombin inhibitor hirudin have suggested that thrombin induces cell proliferation during the process of restenosis, it has not yet been determined whether the thrombin effect is direct or indirect (Sarembock et al., Circulation 1992, 84: 232–243). Despite the implication of the cellular effects of thrombin in a variety of pathological symptoms, no therapeutically active substance is known which specifically blocks such effects.
The thrombin receptor (PAR1) has recently been cloned (Vu et al., Cell, 1991, 64: 1057–1068), opening an important door to development of substances which target cellular thrombin receptors. Detailed examination of the amino acid sequence of this thrombin receptor has revealed a thrombin binding site and hydrolysis site located in the 100 residue amino terminal domain of the receptor. Later research by amino acid mutation in the receptor has established that limited hydrolysis of this portion of the thrombin receptor by thrombin is necessary for receptor activation (Vu et al., Nature, 1991, 353: 674–677). A synthetic peptide corresponding to the amino acid sequence newly generated by hydrolysis of the thrombin receptor (the synthetic peptide is known as “thrombin receptor activating peptide”, or TRAP) can activate receptors which have not been hydrolyzed by thrombin. This suggests that the cleavage of the receptor, the new amino acid sequence generated at the amino terminal (known as the “tethered ligand peptide”) functions as the ligand and interacts with the distal binding site. Further studies of TRAP have confirmed homology of the thrombin receptors present in platelet, endothelial cell, fibroblast and smooth muscle cell (Hung et al., J. Cell. Biol. 1992, 116: 827–832; and Ngaiza, Jaffe, Biochem. Biophys. Res. Commun. 1991, 179: 1656–1661).
Research on the structure activity relationship of TRAP suggests that the pentapeptide Phe-Leu-Leu-Arg-Asn is a weak antagonist for platelet thrombin receptors activated by either thrombin or TRAP (Vassallo. et al., J. Biol. Chem., 1992, 267: 6081–6085 (1992)). Different approaches to receptor antagonism have also been examined by other groups. One of these approaches has been an attempt to prepare antibodies for the thrombin binding domain of the thrombin receptor. Such antibodies specifically and effectively suppress activation of platelets by thrombin, and act as thrombin receptor antagonists (Hung et al., J. Clin. Invest. 1992, 89: 1350–1353). Another approach has been the development of peptide derivatives from TRAP (Steven M. S., J. Med. Chem. 1996, 39: 4879–4887; William J. H., Bioorg. Med. Chem. Lett. 1998, 8: 1649–1654; and David F. M., Bioorg. Med. Chem. Lett. 1999, 9: 255–260). Yet another approach has been the development of low molecular weight compounds discovered by high throughput screening using various assay systems such as receptor binding (Andrew W. S. et al., Bioorg. Med Chem. Lett. 1999, 9: 2073–2078; Scherig Plough WO99/26943; and Halord S. et al., ACS meeting in October 2001).